This is the most common type of spillway provided on gravity dams. The profile of the spillway is ogee or ‘s’ shaped.

The overflowing water is guided smoothly over the crest and profile of the spillway so that the overflow water does not break contact with the spillway surface.

If this is not assured, a vacuum may form at a point of separation and cavitation may occur. In addition to cavitation, vibration from the alternate making and breaking of contact between the water and face of dam may result in serious structural damage.

Hence the upper profile of the ogee is, made to confirm with the lower nappe of a freely falling jet of water over a sharp crested weir, when the flow rate corresponding to the maximum designed capacity of the spillway.

Here, the essential difference between the straight drop spillway and the ogee or overflow spillway should be clearly noted. In the straight drop spillway type, the jet falls clearly away from the face of the spillway and the gap between the jet and face is kept ventilated.

In the ogee or overflow spillway, the falling water is made to glide over the curved profile of the spillway.

A smooth gradual reverse curvature on the downstream face of the spillway is provided. This reverse curve turns the flow on the apron of a stilling basin or into the spillway discharge channel.